(1) Field of the Invention
The present invention relates to a heater control device used in, for instance, a fixing device that fixes, onto a recording sheet, a toner image having been transferred onto the recording sheet, a fixing device including a heater control device, and an image forming apparatus.
(2) Description of Related Art
An electro-photographic image forming apparatus, such as a printer and a copier, is provided with a fixing device that fixes, onto a recording sheet such as a piece of paper and an OHP sheet, a toner image having been transferred onto the recording sheet. As a heater for heating toner transferred onto a recording sheet, such a fixing device includes, for instance, a halogen lamp or a resistance heating element.
As conventional technology related to a heater used in a fixing device, a technology is known of performing phase control of alternating power (hereinafter referred to as “AC” power) supplied from a commercial AC power source and supplying phase-controlled power to a heater used in a fixing device. Here, the phase control is performed to reduce a so-called “inrush current” that is generated upon commencement of power supply to the heater as much as possible.
The phase control as described above involves controlling a phase angle of AC power such that an on-duty ratio gradually increases, and thereby gradually increasing the amount of power supplied to the heater. Here, the term “on-duty ratio” indicates a ratio of a period during which power supply to the heater is performed within a half-cycle of AC power. By performing the phase control as described above, a rapid change in voltage taking place when power supply to the heater is commenced can be suppressed. This further realizes suppressing the generation of flicker in a lighting fixture, etc., that receives commercial AC power via the same power supply line as the heater (refer to Japanese Patent Application Publication No. H10-91037, for example).
However, the phase control as described above, which involves switching on and off the power supply to the heater within each half-cycle of the AC power, leads to a risk of harmonic currents appearing on the power supply line to which the heater and other electric devices are connected.
Such harmonic currents appearing on the power supply line negatively affect the other electric devices connected to and receiving AC power from the same power supply line as the heater, and therefore are problematic. Examples of negative effects that harmonic currents bring about in electric devices include: the degradation of electric parts such as capacitors in the electric devices, and in cases where the electric devices are communication devices in particular, generation of noises and improper displaying of images. In view of such problems posed by harmonic currents, the International Electrotechnical Commission (IEC) has adopted standards related to the restriction of harmonic currents (hereinafter referred to as “harmonic current related standards”). According to such standards, restriction is imposed on harmonic currents such that an average of current values of a harmonic current generated within a predetermined time period equals or falls below a predetermined threshold value.
Here, it should be noted that the phase control disclosed in Japanese Patent Application Publication No. H10-91037 involves simply increasing the on-duty ratio by a fixed amount for each hertz. Thus, when employing the phase control disclosed in Japanese Patent Application Publication No. H10-91037 and setting the fixed amount to a small value, a relatively great amount of time is required until a target power amount is supplied to the heater from the commencement of the phase control (the amount of time required until a target power amount is supplied to the heater from the commencement of the phase control hereinafter referred to as a “through-up time”). The setting of the fixed amount in the phase control disclosed in Japanese Patent Application Publication No. H10-91037 to a small value as described above has both positive and negative effects. On the positive side, the generation of flicker can be suppressed since the change in voltage supplied to the heater is moderated. On the negative side, conformity to the harmonic current related standards as described above cannot be ensured due to harmonic currents being generated over a long period of time.
Similarly, both positive and negative effects as described in the following are brought about when employing the phase control disclosed in Japanese Patent Application Publication No. H10-91037 and setting the fixed amount to a large value. That is, on the positive side, conformity to the harmonic current related standards can be ensured due to a shorter through-up time than the above-described case being realized. On the negative side, the generation of flicker cannot be suppressed since a rapid change takes place in the voltage supplied to the heater, which brings about an increase in inrush current.
Such problems are not unique to a heater provided to a fixing device, but also are observed in heaters in general when power is supplied thereto.